Built-up roofing systems have been used for many years on flat deck roofs. Originally, several layers of tar paper were laid by coating the deck and applying paper strips with hot tar. Later, the tar paper was replaced by layers of felt or fibrous material and the hot tar was replaced by hot asphalt. In either case, each strip of paper or felt was overlapped about three-quarters of the width of the previous sheet so that the completed roof was multi-ply system. In some cases a top layer of crushed stone or gravel ballast was applied to aid in resisting uplift forces.
Originally, no insulation layer was used and the temperature of the air inside a building helped to counter the contrasting outside temperature and kept the membrane within a moderate temperature range. In recent years rising fuel costs have made it desirable, if not necessary, to use a layer of insulation next to the roof deck. As a result the difference between temperatures inside and outside of the building was greatly increased both winter and summer, subjecting the built-up plies to increased temperature ranges resulting in increased expansion and shrinking stresses causing cracking and splitting, and consequent leakage.
The labor cost of installing built-up roofs and the cost of asphalts used in the strip material and in the application thereof have been rapidly increasing in recent years. Consequently, cheaper materials have been used with the inevitable result that such roofs fail prematurely and experience leakage problems within the first five years after installation.
Moreover, the use of hot asphalt produces substantial amount of fumes, creating an objectionable pollution problem violating EPA regulations, and possibly state regulations in certain areas.
Various solutions to these problems have been suggested including the single ply membrane systems which generally utilize elastic rubber or plastic strips of various widths. The rolled strips are unrolled onto the roof deck and attached to the deck by suitable adhesive or mechanical fasteners, or both, and seamed together at the edges. Hot tar or asphalt is not usually required. The membranes being used, of which I am aware, include synthetic elastomers or rubbers, and thermoplastics, reinforced and nonreinforced.
Essentially, three types of single-ply systems employing such membranes are being used. One is a fully adhered system wherein the membrane is adhesively secured to the entire surface area of the roof deck and the edges of the membrane strips or sheets are seamed, usually having their marginal edges overlapped and bonded.
The second type is a so-called loose-laid system in which the membrane is fastened only at the perimeter and held down over the entire area by a layer of ballast amounting to 10 to 15 pounds per square foot, to resist uplift forces.
The third type is a mechanically fastened system in which the membrane is secured to the deck at intervals with mechanical fasteners and the edges of the membrane strips are bonded together.
One variation of a mechanically fastened system utilizes numerous mechanical fasteners to secure the membrane at close enough intervals along the strip edges so that uplift forces are absorbed at the fastening points and no gravel ballast is required. Any of the fasteners which are exposed must be waterproofed at additional cost. Another variation is to use a large number of plastic disks or washers mechanically fastened to the deck at close intervals and apply adhesive to the disks as the membrane is unrolled over them. The adherence of the membrane to the disks is relied upon to withstand uplift and no gravel ballast is used.
The fully adhered single-ply system has numerous advantages over the built-up roof system, including longer life, weatherability and greater integrity due to improved membranes, reduction of maintenance costs and elimination of hot asphalt. However, adhering the membrane to the entire deck surface is a very expensive operation from the standpoint of labor and the high cost of a special adhesive which can be applied without heating. Moreover, most such adhesives can not be applied in high moisture environments or in cold temperatures. The overall cost of the fully adhered single-ply system is generally more than the conventional built-up system. Further, error-free installation is required as errors in one ply are not repaired or covered over by the additional plies as in built-up systems. Other problems include flex fatigue of the membrane and subsequent cracking due to continual stressing of the material in isolated areas due to daily expansion and contraction of the deck.
The loose-laid single-ply system using a top layer of ballast eliminates the material and labor cost of the special adhesive but is still quite labor intensive. Moreover, many existing roofs have not been built strong enough to carry the additional weight of the ballast in addition to the other loading requirements. Accordingly, new roofs must be designed to support the required weight of the ballast amounting to 10 to 15 pounds per square foot in addition to the other loads. Further, the ballast tends to concentrate the stress loads due to expansion of the roof deck, as in some areas the ballast does not move during expansion and contraction of the building, again concentrating the stress and causing flex fatigue of the membrane and premature failure.
Mechanically fastened single-ply systems without gravel ballast on top reduce the concentration of stress loads on the membrane, but in order to withstand the uplift forces the membrane must be fastened to the deck at close intervals over the entire area, minimizing the areas of membrane not secured to the deck. If the strips of membrane are secured only along the bonded side edges, the width of the strips should be restricted to a dimension (about 5-6 ft.) in order to ensure adequate resistance to uplift in the membrane between fastening locations.
Mechanically fastened single-ply systems utilizing plastic disks fastened to the deck at close intervals and coated with adhesive in situ as the membrane is unrolled over them require additional labor in applying the adhesive, as well as the added cost of the adhesive. In addition, the reliability of the adhesive can be decreased by long term heat and moisture exposure, causing premature wind uplift failure.